1. Field of the Invention
The present invention relates to video and audio distribution systems and more particularly concerns improved distribution of numbers of channels of video, audio and other information.
2. Description of Related Art
Each passenger of a passenger aircraft may be provided with an individually controllable electronics box unit to enable personal selection of individual signals from among a group of different audio signals, a group of different video signals, and in some cases digital data for graphics, slide shows and the like. The audio signals and the video signals, together with their own audio, and also the data signals, are transmitted to each of the passenger seats from one or more central audio and/or video or data sources.
In the past, audio and data signals have been transmitted utilizing twisted pair wiring. Present systems are limited to about thirty channels of audio because data on twisted pair wiring is limited to relatively low frequencies. This limits data transmission rates, and, therefore, the number of audio channels. The video distribution system employs coaxial cable for distribution of video signals in the frequency range of 50 to 300 megahertz carriers.
The aircraft cabin entertainment system currently provides approximately 24 channels of audio, a public address channel, and a single channel for video audio. These audio signals are converted to digital signals and multiplexed onto a twisted wire pair at data rates up to 15 megabits per second. No video signals are placed on this wire. It is desirable that aircraft cabin entertainment systems be upgraded to include many channels of video distributed to passenger seats by coaxial cable. Each video channel may have mono or stereo audio and may transmit audio in several languages. As a result, the number of audio channels may increase sharply from the present 20 to 30 channels to 60 to 100 channels.
At present, audio signals are transmitted by means of one of two methods. The video channel audio can be transmitted together with the video just as in a conventional television channel. Alternatively, the video channel audio can be separated from the video and transmitted on an analog FM carrier employing one FM channel for each video audio and one FM channel for each music or other audio signal. The first approach requires an audio demodulator at each seat for the video audio and requires special circuitry to select the video audio as distinguished from the music and other audio. The second approach requires an FM demodulator at each seat. Further, in this second approach, utilization of as many as 100 separate RF carriers is likely to cause RF interference. Neither approach provides for addition of data services, such as graphics and slide show presentations, as passenger interests and demands may change in the future.
Thus, the methods of transmission of combinations of audio and digital signals are subject to a number of limitations, which would prevent use of economically and technically feasible circuitry for transmission of increased numbers of entertainment channels.
Furthermore, multiplexing together as many as 60 to 100 audio signals into a 60 megabit per second data stream, as could be required by prior systems, would generate a frequency spectrum of at least 300 megahertz of bandwidth, causing significant interference with the video signals. The resultant signal would require expensive amplifiers to pass the frequency spectrum extending from DC to 400 megahertz. Such a wide bandwidth imposes still further problems due to the frequency versus loss characteristics of coaxial cable. These characteristics, often known as "tilt", generate greater losses at higher frequencies than at lower frequencies, so that expensive techniques for equalization of signal amplitudes at the utilization station would be required.
Accordingly, it is an object of the present invention to provide an entertainment signal distribution system that avoids or minimizes above mentioned problems.